By providing us with additional information about something, relative clauses save us from having to start a new sentence. We use relative pronouns to refer to various entities: ‘who’ for people, ‘which’ for things, and ‘that’ for people or things.
Which pronoun it is correct to use depends on various factors, including whether they introduce a defining or a non-defining relative clause. Defining relative clauses are used to give vital details about someone or something - details that are necessary to understand who or what is being referred to. Non-defining relative clauses are used to give extra information that is not essential.
Who: form and uses
‘Who’ is the subject or object pronoun for people. We use it instead of he/she/they if it is acting as the subject, and in place of her/his/them if we are referring to an object. For example:
I told you about the man who taught me French. (= 'who taught me French' tells us which man)
The police arrested a man who David worked with. (= 'who David worked with’ tells us which man)
You met a woman who I went to school with. (= ‘ who I went to school with’ tells us which woman)
Where the relative pronoun is the subject or object of a defining relative clause, we can use either ‘who’ or ‘that’ for people, as demonstrated here:
I’m looking for an equestrian who/that can ride well.
She has a daughter who/that is a nurse.
The same is not true of non-defining relative clauses, where we must always use ‘who’ to refer to a person, as demonstrated below:
My boyfriend, who is very handsome, lives in Manchester.
My sister, who I’m very close to, likes shopping.
Which: form and uses
‘Which’ is the subject or object pronoun for animals and things. For example:
Did you see the dog which had escaped into the road? (= 'which had escaped into the road' tells us which dog)
Did you see the kite which flew up into the air? (= 'which flew up in the air’ tells us which kite)
Where the relative pronoun is the subject or object of a defining relative clause, we are able to use either ‘which’ or ‘that’ for animals and things, as exemplified here:
We purchased a property which/that is 500 years old.
She loves the jam which/that I sent her.
The rule is different where non-defining relative clauses come into play, in which instance we must always use ‘which’ to refer to things, as illustrated below:
My car, which I’ve had for over a decade, is a real rust bucket.
My childhood home, which is in Swansea, is very old and beautiful.
That: form and uses
‘That’ works a little differently to the other two, in that it can be used interchangeably with either of them in a defining relative clause, but cannot be used at all in a non-defining relative clause. For example, we can say:
The bike that I loved was stolen from outside the coffee shop. (= 'that I loved' tells us which bike)
The university that she went to is very famous. (= 'that she went to’ tells us which university)
But we cannot say:
My former boss, that is very nice, moved abroad.
Relative pronouns as the object
When using the relative pronouns examined above, there is one further rule that you must be aware of: where they are the object of the clause, they can be dropped.
She loves the chocolate which/that I bought her.
...could be changed into...
She loves the chocolate I bought.
…without any loss of meaning.
Understanding preposition placement in relative clauses
We must also take care to understand how relative clauses and prepositions work together.
Prepositions are often found in relative clauses, with a relative pronoun being the object of the preposition.
As a rule, the preposition is usually found at the end of this, and the pronoun can be either included or omitted.
In formal English, however, the position changes, and the preposition is found before the relative pronoun, which cannot be omitted.
Everyday English: Does she know the boy that Jemima is speaking to?
Formal English: Does she know the boy to whom Jemima is speaking?
Everyday English: The woman who he is talking with is the head of a large corporation.
Formal English: The woman with whom he is talking is the head of a large corporation.